Fix linux-ia32 prebuild resolution falling back to linux-x64

[Copilot]

The Test (linux-ia32) Actions job fails because native module loading resolves to prebuilds/linux-x64/pty.node instead of the 32-bit Linux target, causing module load failure at runtime. This PR tightens prebuild path resolution so linux-ia32 selects the correct artifact and only falls back to local build outputs when appropriate.

• Root cause addressed: architecture-specific prebuild selection

  • Ensure the computed prebuild directory matches the effective runtime architecture for Linux ia32.
  • Prevent incorrect linux-x64 lookup in ia32 CI/container contexts.

• Loader fallback behavior

  • Keep existing Release/Debug fallback semantics for source builds.
  • Preserve prebuild-first behavior while avoiding cross-arch mismatches.

• Targeted code path updates

  • Minimal changes in prebuild path computation and native loader selection logic.
  • No unrelated behavior or platform handling changes.

// before: could resolve to wrong prebuild dir in linux-ia32 job
const pathToBuild = `../prebuilds/${os.platform()}-${os.arch()}/${prebuildName()}`;

// after: resolve using normalized/effective arch for runtime target
const prebuildArch = resolveRuntimeArch(); // returns 'ia32' in linux-ia32 context
const pathToBuild = `../prebuilds/${os.platform()}-${prebuildArch}/${prebuildName()}`;